To peel either poblano chiles or green bell peppers, hold over flame of the burner with a long wood-handled fork or grill under the broiler, turning frequently until the skin in brown and blistered.
Immediately wrap in foil or place in paper bag.
Close tightly and let steam for 5 to 10 minutes.
Skin will loosen and can be easily rubbed or stripped off.
Discard seeds and white ribs.
Puree peeled, roasted peppers with unpeeled hot green chilepeppers in a blender of food processor, adding a little broth if needed (if you have a hand blender, just cut up peppers, add then to soup, and purée)
Melt butter in a 2-quart saucepan and sauté onion and garlic until wilted but not taking on color, about 5 minutes.
Blend in flour with a whisk over low heat, then add pureed chilies and soup.